Continuous ink jet printing systems are known to be of use in the industry of high speed digital printing. In many installations a plurality of ink jet print heads are arranged inline over a web of paper to produce multi-color images in a single pass. In a typical printing system, to facilitate the digital printing process the ink jet print heads generate an array of micro-droplets of ink in the cross-web direction at a resolution of 300 drops per inch and at a rate of 100 Khz. The timing of the release of each cross-web pixel of droplets is computer synchronized to tachometer pulses received from the web transport system such that pixels will be released (printed) at a resolution of 300 or 600 drops per inch in the web travel direction. The tachometer pulses are also used to delay the printing of successive print heads such that the successive print head data will overlay (register) on top of the previous print head data; therefore, combining the two (or more) colors of ink from the print heads to achieve multi-color images. Inherently, this color-to-color registration process requires input from some expensive and complex electronic vision system or a system operator to observe the nominal registration error and then correct electronically as required to “zero” the average color-to-color registration error.
In addition to the average color-to-color registration error adjustment, all web transport systems contain motor servo systems that create web speed fluctuations that result in web stretch (tension) variations which in turn equates to color-to-color registration error variation around the error average. Therefore, the need arises for good color-to-color registration measurement capability—certainly for operator ease of average adjustment but more importantly, for identifying and aiding in reducing the magnitude of error variations.
Previous conventional art has led to the use of registration marks typically located at the corner(s) of each image. Marks have varied from simple lines, crosshairs, circles, or squares to combination crosshairs/circles, lines at various angles, chevron patterns, and others. While most of these approaches can be used to enable an operator to adequately adjust the average registration error near zero, the ability to use them to evaluate registration variation is marginal. Past procedures for evaluating color-to-color registration error variation in continuous ink jet printing systems utilized these marks at the corners of each image but in order to get just a minimal amount of data for statistical analysis a sample of at least 25 images would be necessary. The image registration marks would then have to be measured under magnification. At 12″ image spacing this yields registration error data for 25 pixels over of a span of 90,000 pixels—a sampling rate of about 0.03%. Additionally, multi-color text imaging printed under normal conditions has been observed to vary in registration error at multiple cycles per 12″ image. It is seen then that there is a need for a means of viewing registration error as the images are printed and at a sampling rate much greater than any error variation frequencies that may exist